1. Field of the Invention
The present invention relates to an apparatus and method for generating a filtered composite signal obtained by linearly filtering an original composite signal which can be divided into a plurality of sequentially arranged sub-signals, and more particularly to a apparatus and method for generating a composite signal by selectively reading out linearly filtered sub-signals previously stored in memories after linearly filtering an original composite signal, which can be divided into a plurality of sequentially arranged sub-signals, and by combing the sub-signals to output.
2. Description of Prior Arts
In a conventional prior art, such an original composite signal as above is treated with a linear filter in advance of being stored in a memory such as a ROM, and is read out from the memory when the filtered composite signal is to be output.
Referring to FIG. 1, a conventional signal generator as mentioned above will be described below. In FIG. 1, reference numeral 1 denotes a memory, reference numeral 2 a pattern selecting circuit, reference numeral 3 a counter, and reference numeral 4 a clock generating circuit. In the conventional signal generator, the original composite signal containing a plurality of original sub-signals in series is not divided and is treated as a unit with a linear filter before storage in the memory 1. Further each of the sub-signals of the composite signal is a selected one of a plurality of kinds, in other words, a plurality of patterns. Accordingly, the memory 1 stores a plurality of filtered composite signals as unit.
When one of the filtered composite signals is generated, higher (or upper) bits of the addresses of the memory 1 are assigned by the pattern selecting circuit 2 in accordance with the filtered composite signal to be generated, while the remaining lower bits of the addresses of the memory 1 are sequentially assigned by the counter 3 for counting clocks from the clock generating circuit 4, whereby data or the components of the filtered composite signal are sequentially read out from the memory 1.
In the conventional signal generator, assuming that the original composite signal constitutes of two sub-signals respectively containing I.sub.1 and I.sub.2 patterns, the total number of the composite signals is I.sub.1 .times.I.sub.2. The memory 1 thus needs a capacity sufficient to store data samples of I.sub.1 .times.I.sub.2 composite signals.
To describe this in details with reference to FIG. 2, assuming that as shown in FIG. 2(a), an original composite signal x(n) [n: sampling timing] or an input signal to a linear filter (not shown) is expresses as a combination of sub-signals x.sub.1 (n) and x.sub.2 (n); x(n)=x.sub.1 (n)+x.sub.2 (n), namely EQU x(n)=x.sub.1 (n) when n=O-N.sub.1 -1, and EQU x(n)=x.sub.2 (n) when n=N.sub.1 -N.sub.1 +N.sub.2 -1
In this case, a filtered composite signal y(n) or output signal from the linear filter is shown as in FIG. 2(b). In order to make the description simple, it is assumed that the linear filter used is a linear phase finite impulse response (FIR) digital filter having M taps and that the group delay is t. In a case where the filtered output signal y(n) is stored in the memory 1, the number S.sub.1 of data samples is expressed as follows; EQU S.sub.1 =(N.sub.1 +N.sub.2)+(M-1) (1)
Therefore, a memory capacity is needed which is sufficient to store at least data equal to the S.sub.1 samples. In Expression (1), (M-1) is, as shown in FIG. 2(b), data newly generated by virtue of the filter treatment.
In addition, in a case where x.sub.1 (n) and x.sub.2 (n) of the input signal have I.sub.1 and I.sub.2 changeable patterns, respectively, the number of combinations resulting from those patterns becomes I.sub.1 .times.I.sub.2, and therefore in order to store all of the output signals y(n) that have been linear-filter-treated, the number S.sub.1 of samples is represented by Expression (2); EQU S.sub.1 =I.sub.1 .times.I.sub.2 {(N.sub.1 +N.sub.2)-(M-1)} (2)
Thus, is is necessary for the memory 1 to have a capacity obtained by multiplying the capacity in the case that each of the patterns of the sub-signals is only one, by I.sub.1 .times.I.sub.2.
Moreover, when the values of I.sub.1 and I.sub.1 increase, a memory capacity is required to be extremely large, because the number S.sub.1 of samples is proportional to the product of those values. Thus, the conventional signal generator is required to include a memory having a huge storage capacity.
Accordingly, an object of the present invention is to provide an apparatus and method for generating a linearly filtered composite signal which can use a memory having a capacity that is not so large as the one required by a prior art, thereby eliminating problems inherent in the prior art.